[mpiwg-abi] meeting April 4 to discuss integer and handle constants

[Jeff Hammond]

Correction:

The meeting is 15-16 Helsinki, not 17-18.  8 AM New York time was correct.

On Mon, Apr 3, 2023 at 7:57 PM Jeff Hammond <jeff.science at gmail.com> wrote:

> The meeting is tomorrow, Tuesday, April 4, 17-18 Helsinki time, 8-9 AM New
> York time.
>
> The topics / decisions of interest are:
>
> A. Integer constants
>
> We have constants that must be powers of two (mode constants),
> constants that must have a < relationship (error codes and thread levels),
> constants that should be negative to avoid conflicting with e.g. ranks, and
> constants that can be anything.
>
> I have a proposal for constants here:
> https://github.com/mpiwg-abi/specification-text-draft/blob/main/IntegerConstants.md.
> I have not yet implemented Dan's suggestion to make constants unique
> (presumably, for the aforementioned categories that allow it).
>
> I would like feedback on the following:
> 1. Are the values of MPI_MAX_*** acceptable?  In all cases, I chose the
> greater of MPICH and OMPI.  For MPI_BSEND_OVERHEAD, I used 512 to be safe.
> Is that too large?  Is 128 better?
> 2. Are there any opponents to Dan's suggestion that all the constants are
> unique, within reason?  If the consensus favors this, I'll redo all the
> constants accordingly.
> 3. Are there any other integer constant values that bother people?  I have
> very little attachment to any of them, and it is trivial to change them now.
>
> B. Handle constants
>
> I implemented a Huffman code for these (
> https://github.com/mpiwg-abi/specification-text-draft/blob/main/HandleConstants.md
> ).  I made a Python code
> <https://github.com/mpiwg-abi/specification-text-draft/blob/main/print-handle-constants.py>
> that implements it, and the program dumps all the values, and can be
> modified easily to generate mpi_abi.h.
>
> Does anyone oppose the idea of a Huffman code?  I know Hui is indifferent,
> which is obviously fine.  One can ignore the Huffman code and just view the
> results as some random values I chose :-)
>
> If you like Huffman codes, but dislike mine, then please complain soon.
> There are some parts that I do not love.  For example, fixed size types are
> handled consistently and encode their size (as the log_2 of bytes) in bits
> 3:5, while language default types are on a different branch and encode
> their size in bits 8:10.  I can makes those consistent, but it means the
> code branches aren't sequential in the bit indices.  I think that's fine,
> but I am new to Huffman codes.
>
> An alternative to the above is to say that in mpi_abi.h, that MPI_INT,
> MPI_LONG and MPI_LONG_LONG are aliased to MPI_INTn_T according to the ABI
> definition, and do not exist on their own.  This has some appeal, but will
> change the results of MPI_Type_get_name.  What do people think about this?
>
> The good news is there is lots of free space in the Huffman code for new
> handle types and new constants.  I am not worried about running out of
> space.  Already, I have reserved space for a bunch of types that are likely
> to exist in C and C++ in the near future, so those will be trivial to add
> later.
>
> Finally, as noted on Slack, we have to figure out whether we reserve space
> for or standardize some types that OMPI defines today in the MPI_
> namespace.  This is the lowest priority for me right now, so if we don't
> address it this week, that is fine.
>
> Thanks,
>
> Jeff
>
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> ____________________________________________
>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>


-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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